Field of the Invention
The present disclosure relates to an air flow velocity calculating device of an internal combustion engine having a spark plug with a center electrode and a ground electrode placed in a cylinder, and an air flow controlling valve placed in an air intake passage.
Description of the Related Art
An internal combustion engine measuring an electric discharge period of a spark plug on the basis of a change of voltage of a center electrode of the spark plug, is known. The internal combustion engine is described in Japanese Patent Application Publication No. 2009-030515 (JP-A-2009-030515). It is described that there is a one-to-one relationship between the electric discharge period of the spark plug and velocity of air flow near the spark plug, and the velocity of the air flow near the spark plug monotonously decreases in accordance with increase of the electric discharge period, in JP-A-2009-030515. Also, it is described that the electric discharge period corresponds to strength (velocity) of rotational flow in a combustion chamber, in JP-A-2009-030515. It is described that when high voltage is impressed and breakdown occurs between electrodes of the spark plug, an electric discharge occurs and then the electric discharge is sustained, in JP-A-2009-030515. Also, it is described that when the electric discharge occurs, current flows between the electrodes of the spark plug, and then the voltage between the electrodes rapidly decreases. It is described that after the electric discharge is blown out by the air flow, the electrodes of the spark plug are insulated again, and then the voltage between the electrodes rapidly increases, in JP-A-2009-030515. Also, it is described that the electric discharge period can be obtained by monitoring the voltage of the center electrode, by detecting two peaks in time series change of the voltage of the center electrode, and by measuring time between the two peaks, in JP-A-2009-030515.
Typically, if the spark plug ignites once during one cycle of the internal combustion engine, a second electric discharge between the center electrode and the ground electrode of the spark plug is executed, after a first electric discharge between the center electrode and the ground electrode is intercepted by air flow in the cylinder, and then electric discharges between the center electrode and the ground electrode are intermittently repeated, until energy is consumed and the electric discharges cannot be executed.
Typically, velocity of the air flow in the cylinder can be calculated on the basis of an electric discharge sustaining period. The first electric discharge between the center electrode and the ground electrode is sustained during the electric discharge sustaining period. The electric discharge sustaining period is a period from when the first electric, discharge begins to when the first electric discharge is intercepted by the air flow in the cylinder. However, it is difficult to calculate the velocity of the air flow in the cylinder on the basis of an electric discharge sustaining period of second or subsequent electric discharge, because electromagnetic energy of a spark coil is consumed while the first electric discharge is executed.
As mentioned above, the spark plug ignites only once during one cycle of the internal combustion engine. Accordingly, if an irregular phenomenon occurs while the first electric discharge between the center electrode and the ground electrode is executed, the velocity of the air flow in the cylinder cannot be precisely calculated on the basis of the electric discharge sustaining period in which the first electric discharge is sustained. An example of the irregular phenomenon is a rapid change of a direction of the air flow.